1. Field of the Inventive Concepts
The inventive concepts disclosed herein are generally directed to medical implants, and more particularly, but not by way of limitation, to a vertebral body replacement apparatus configured to be at least partially dynamic when implanted into a spine.
2. Brief Description of Related Art
The human spinal column, or spine, is highly complex, in that it includes over twenty bones coupled to one another so as to support the body and to house and protect critical elements of the nervous system. In addition, the spine is a highly flexible structure, capable of a high-degree of curvature and twist in multiple directions. The bones and connective tissues of an adult human spine are coupled sequentially to one another by a tri-joint complex which consists of an anterior joint between vertebral bodies, and two posterior facet joints. The vertebral bodies of adjacent vertebrae are separated and cushioned by cartilage spacers referred to as intervertebral discs. The vertebral bones of the spine are classified as cervical, thoracic, lumbar, and sacral. The cervical portion of the spine, which includes the upper portion of the spine up to the base of the skull, is the most flexible of all the regions of the spinal column, and includes the first seven vertebrae. The twelve intermediate bones comprise the thoracic vertebrae, and connect to the lower spine which comprises the five lumbar vertebrae. The base of the spine includes the sacral bones (including the coccyx).
A typical human vertebra consists of two essential parts: an anterior (front) segment, which is the vertebral body; and a posterior (back) segment—the vertebral (neural) arch—which encloses the vertebral foramen. The vertebral arch is formed by a pair of pedicles and a pair of laminae, and supports seven processes—four articular, two transverse, and one spinous.
The vertebral body is the largest portion of the vertebrae and is generally cylindrical in shape. Vertebral bodies have upper and lower surfaces, which are generally flat or slightly concave. The surfaces are roughened to allow for the attachment of the intervertebral discs. The vertebral bodies and the intervertebral disks cooperate to provide structural support to the spinal column, with the intervertebral disks cushioning the vertebrae and absorbing and adapting to forces exerted on the vertebral bodies.
In some cases of spinal injuries, the forces exerted on the spinal column are so great, as to cause a partial or complete fracture or collapse of one or more of the vertebral bodies, and significant damage to the intervertebral disks surrounding the fractured or collapsed vertebral body. A vertebral body fracture or collapse may also be caused by osteoporosis, arthritis, tumors, or other diseases.
Regardless of the cause, it is difficult for the damaged vertebral body and intervertebral disks to heal due to the constant forces exerted on the spinal column, and/or due to disease progression. Further, due to bulging or displaced damaged vertebral body fragments or intervertebral disks, pressure may be exerted on the spinal cord, or other neural tissues surrounding the damaged vertebral body or intervertebral disks, which may lead to significant pain, neurological damage, and even paralysis in some severe cases.
A surgical procedure called vertebral body replacement (VBR) has been developed to remove the damaged vertebral body and intervertebral disks, and to replace them with an implantable VBR apparatus, such that the proper height, alignment, and curvature of the patient's spinal column are maintained or are not significantly compromised.
VBR is generally performed by locating the damaged vertebral body (e.g., with medical imaging) and accessing it via an appropriate surgical incision. Once the vertebral body is accessed, surgical tools may be used to remove the damaged portion or the majority of the vertebral body and the two intervertebral disks surrounding the damaged vertebral body, such that the lower surface of the vertebral body above and the upper surface of the vertebral body below the removed vertebral body are exposed.
Next, a generally cylindrical VBR apparatus of appropriate size is selected and inserted in the location of the removed vertebral body. The VBR apparatus generally has endplates which contact the exposed lower surface of the vertebral body above the removed vertebral body, and the exposed upper surface of the vertebral body below the removed vertebral body. The endplates are configured to engage the VBR apparatus with the two adjacent vertebral bodies and to keep it in place once implanted. The design, shape, and angle of the endplates that contact the adjacent vertebrae are selected to ensure proper spinal height, alignment, and curvature, and to securely attach the VBR apparatus to adjacent vertebrae, such that the VBR apparatus does not become dislodged, or otherwise displaced post-implantation.
Some existing VBR apparatuses allow surgeons to adjust the height of the VBR apparatus to match the original height, alignment, or curvature of the patient's spine, and some VBR apparatuses have a porous hollow body or cage, which allows surgeons to insert a bone graft into the VBR apparatus. The bone graft may eventually grow through, or around, the VBR apparatus, and may fuse the two vertebrae that are in contact with the VBR apparatus over time.
In some cases, one or more supplemental fixation devices, such as stabilizing rods, plates, or bone screws, may be attached to the vertebrae above and below the VBR apparatus and/or to the VBR apparatus to absorb some of the forces exerted on the VBR apparatus and/or to provide additional stabilization of the spine while the bone graft is fusing the two vertebrae together. If the VBR apparatus is a bone-fusion device, over the next several months the bone graft grows into, or around, the VBR apparatus to eventually fuse the adjacent vertebral bodies together. If the VBR apparatus is a non-fusion device, the supplemental fixation devices and the VBR apparatus may function to replace the removed vertebral body and the VBR apparatus and adjacent vertebrae may not be fused together.
However, existing VBR apparatuses suffer from several disadvantages. For example, existing VBR apparatuses are generally rigid and inelastic devices and have bone-contacting surface designs which, due to local patient anatomies and angulation, may result in concentrating a large amount of force onto a small area on the prior art VBR apparatus bone-contacting surfaces. This is referred to as point-loading and may increase the chances of adjacent vertebral body subsidence and VBR apparatus failure.
Further, rigid existing VBR apparatuses remain static (e.g., inelastic in an axial and/or lateral direction) once implanted and transfer strain away from the graft inside the VBR apparatus and onto adjacent vertebral bodies contacted by existing VBR apparatuses. Bone remodeling is controlled by peak strain, and when a bone is subjected to just a few cycles per day of strain above a certain level, the bone is maintained and/or new bone formation occurs the strengthen the bone. In the case of fusion VBR apparatuses, it would be advantageous to provide a substantially elastic VBR apparatus that transfers some strain and/or other forces to the graft material to stimulate the graft to fuse the adjacent vertebra more quickly than prior art implants.
Some attempts have been made in the prior art to include some elasticity in the endplates of VBR apparatuses, such as the endplates described in PCT patent application Ser. No. PCT/US2012/65287, filed on Nov. 15, 2012, and in U.S. Pat. No. 8,252,059, the entire disclosures of which are expressly incorporated herein by reference. However, such elasticity has been limited to the endplates or to disk-replacement devices, while existing VBR apparatuses have remained inelastic and static.
Accordingly, a need exists in the prior art for a VBR apparatus configured to remain at least partially dynamic in a lateral and/or axial direction when implanted into a spine, to more efficiently distribute and absorb forces applied to the VBR apparatus and to stimulate bone graft growth. It is to such VBR apparatuses that the inventive concepts disclosed herein are directed.